We tend to think of type 2 diabetes as a disease that afflicts people who are overweight. But it can also appear in people with perfectly healthy weights—and be more deadly in them. A study published today in the Journal of the American Medical Association indicates that normal-weight people diagnosed with type 2 diabetes have double the risk of dying from heart disease and other causes than overweight people with diabetes.

Such apparent “protection” by excess weight has been called the obesity paradox. It’s been seen with other conditions, like heart failure and end-stage kidney disease. Overweight or obese people with these conditions seem to fare better or live longer than their normal-weight counterparts.

That doesn’t mean gaining weight is a healthy strategy. Instead, it probably means that something else besides weight—like the amount of fat around the waist—may be contributing to the onset and severity of type 2 diabetes.

Diabetes types

There are two basic types of diabetes. Type 1 diabetes occurs when the body stops making insulin. This happens when the immune system mistakenly attacks insulin-making cells in the pancreas. Without insulin, cells can’t absorb sugar (glucose) from the bloodstream. The resulting high sugar levels in the blood damages nerves, arteries, and other tissues. Type 1 diabetes often appears early in life, but can happen later. People with type 1 diabetes must take insulin.

Type 2 diabetes occurs when cells become resistant to insulin’s “open up for sugar” signal. Exactly why this happens is still something of a mystery. But excess weight contributes to it, since fat cells affect how the body uses glucose and produces insulin. Lack of physical activity also plays a role. Medications that make muscle and other cells more responsive to insulin can control type 2 diabetes. In some cases all that’s needed is more exercise and careful attention to diet.

Waist size matters

Somewhere in the neighborhood of 85% of people who develop type 2 diabetes are overweight or obese. So carrying too many pounds won’t protect you against this potentially life-shortening condition. How then to explain that excess weight is linked with fewer cardiovascular complications, like heart attack and stroke?

The researchers used body-mass index (BMI) to assign people to weight categories. (You can calculate your BMI here.)

Normal: 18.5 to 24.9

Overweight: 25 to 29.9

Obese: 30 or greater

But BMI can be misleading, especially for people in the overweight category. BMI does not always reflect the amount of body fat a person carries. In addition, where body fat accumulates is just as important, and may be even more important, than how much body fat a person has.

This is where waist size comes in. Waist size correlates with belly fat, what doctors call visceral fat. The bigger your waist, the more belly fat you are likely to have. And it’s belly fat that increases your risk of heart disease, stroke, and other blood vessel disorders, not to mention your risk of developing type 2 diabetes.

In the JAMA study, the researchers found that people who were overweight or slightly obese but who had reasonable waist sizes (and therefore not too much belly fat) had better prognoses. In contrast, normal-weight people with large waists had the highest risk of premature death.

Muscle mass also matters. Muscle burns sugar. The more muscle mass you have, the less sugar is converted to fat. BMI does not provide information on how much muscle, or fat, you carry.

Put your muscles to work

BMI and waist size are both important indicators of health. A BMI of 30 or higher indicates obesity and clearly needs action, independent of waist size. However, for people with BMIs between 18.5 and 29.9, I prefer to use waist size. In this range, women should aim for a waist size less than 32 inches; men should aim for 35 inches or less.

If you are short or tall, height can make a difference when evaluating your waist size. One rule of thumb for finding your maximum waist size is to divide your height by two. For example, if you are 6 feet tall (72 inches), half of that is 36 inches. That would be your upper limit.

This study and others reinforce for me the value of strength training. Lifting weights or working out on a muscle-resisting exercise machine can build, or at least maintain, muscle mass. Active muscles can help prevent type 2 diabetes or control it if it appears. Current recommendations call for doing resistance exercises at least twice per week. I think that every other day is ideal.

If you have diabetes and your BMI was less than 25 when you were diagnosed, weight training is essential no matter what your waist size.